Molecular dynamics is employed to investigate carbon diffusion in cementite. An approximation that carbon atoms can interact with each other only indirectly (via neighbouring iron atoms) is used. The interstitial mechanism of carbon diffusion in cementite is elucidated. The formation energy of defects (a carbon atom on an interstitial position and a vacant site on a regular carbon position) as well as the migration energy of carbon atoms are estimated in the temperature range 1273–1373 K.